The present invention relates to a door opening and closing mechanism utilizing a fluid actuator such as a pneumatic cylinder. More specifically, the present invention relates to such a mechanism which may be used in opening and closing very heavy doors, such as sliding doors found in penal institutions, as well as in other applications.
Pneumatic cylinders have been utilized to operate sliding doors in penal institutions. However, existing pneumatic cylinder based systems are subject to a number of drawbacks. In a typical prior art approach, while the door is stationary, pressurized air is not supplied to either side of a piston utilized in shifting the door between open and closed position. Because of this lack of air pressure, each side of the piston is typically at atmospheric pressure (e.g. 14 psi) and the door is not moved. To shift the door, relatively high pressure air (e.g. 100 psi) is applied to one side of the piston while the other side of the piston communicates through a choke orifice to the atmosphere. Upon the application of this high pressure, the door moves relatively rapidly and eventually compresses the air at the low pressure side of the piston, due to the fact that the air cannot escape through the choke orifice very rapidly. This compression of air provides some dampening of door movement as the door approaches the open or closed positions under these conditions. If, however, after moving the door in one direction by pressurizing a first side of the piston it is desired to immediately reverse the direction of door movement, air under pressure is then applied to the opposite or second side of the piston. However, both sides of the piston are then approximately at the relatively high pressure (e.g. 100 psi) because the pressure at the first side of the piston is relieved slowly through the choke orifice. Consequently, the door initially may not move at all in the reverse direction. Thereafter, the door tends to move in the reverse direction very slowly with the velocity of the door being limited by the rate which air bleeds from the first side of the piston through the choke orifice. The door under these condition moves at a relatively constant speed as the bleeding takes place, but not at a rate which is fast enough for the piston to compress air as the door approaches its end position (either open or closed as the case may be). Consequently, in this case the door tends to noisily hit a doorjamb or stop. Also, the noise of air bleeding through choke orifices is significant. In addition, as explained above, it is difficult, if not virtually impossible, to immediately reverse the direction of travel of the door due to the relatively high air pressure at both sides of the piston.
Pivot type doors operated by electric motors are also known to slow down the rate of closing of a door as the door approaches a closed position. However, electrical motor operated sliding doors known to the inventors and used in penal institutions open and close the doors at one speed, and utilize a clutch which slips when the doors hit a stop or doorjamb. Not only do these doors lack fluid actuators and the advantages thereof, they tend to be noisy as well.
Therefore, a need exists for an improved fluid actuator based door operating mechanism and for a door assembly with such a mechanism which is designed to overcome these and other disadvantages of the prior art.